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- 저자 : Rakesh Kumar Dwivedi (검색결과 2 건)
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Anomalous Weak Ferromagnetism in Electron-doped Nd_(1−x)Sr_xMnO_3 (0.50 ≤ x ≤ 0.62) Thin Films
Pawan Kumar,Ravikant Prasad,Hari Krishna Singh,Rakesh Kumar Dwivedi,Avanish Kumar Srivastava,Mangala Prasad Singh 한국물리학회 2011 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.59 No.4
In the present work we demonstrate that an anomalous weak ferromagnetism occurs in the composition range 0.50 ≤ x ≤ 0.62 in nanostructured thin films of Nd_(1−x)Sr_xMnO_3 (NSMO). Nanocrystalline thin films of overdoped manganite Nd_(1−x)Sr_xMnO_3 (<I>x</I> ∼ 0.50, 0.55, 0.60 and 0.62) on single crystal LaAlO_3 (001) substrates are grown by nebulized chemical spray pyrolysis technique. These single phase films possess average crystallite size ∼15 nm and the width of the grain boundaries is ∼1 - 2 nm. In the composition range 0.50 ≤ x ≤ 0.62, the ground state of NSMO is A-type antiferromagnetic (AFM) metallic. However, in the present films a broad paramagnetic (PM) to ferromagnetic (FM) transition is observed in all the compositions at T_C ∼ 226 K for x = 0.50 and 235 K for rest of the samples. All the films show a well-defined M-H hysteresis loop at 5 K. The coercivity (H_C) of these films is much larger than those having composition range 0.35 < x < 0.45. For x = 0.50 (0.62) film H_C = -1210 (-1484) Oe and +1162 (+1476) Oe. The asymmetry in coercivity suggests the presence of weak exchange bias effect in these films. It is observed that the FM ground state in these films have smaller magnetic moment per Mn atom than the expected full moment using the rigid model, we term this as a weak ferromagnetic (WFM) state arising due to the destabilization of the AFM ordering. We propose a possible scenario based on the combined effect of spin reorganization and enhanced orbital disordering in nanosized manganites, to explain the observed anomalous weak ferromagnetism in the A-type AFM spin ordered regime.
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Pawan Kumar,Rakesh Kumar Dwivedi,Pawan Kumar,Ravikant Prasad,Praveen Kumar Siwach,Hari Krishna Singh 한국물리학회 2011 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.58 No.1
Thin films of La_(0.80)Sr_(0.20)Mn_(1−x)Cu_xO_3 with x = 0.00, 0.04, 0.08, 0.10, 0.15, and 0.20 were deposited on single-crystal LaAlO_3 (100) (LAO) substrates at 250 ± 10 ℃ by using a nebulized spray pyrolysis technique. Structural, surface, and transport characterizations were done by X-ray diffraction (XRD), atomic force microscopy (AFM) and the standard four probe technique, respectively. XRD patterns revealed the formation of single-phase polycrystalline films with a rhombohedral crystal structure. Cu substitution resulted in a uniform decrease in the metal insulator transition (T_(IM)) temperature up to x = 0.10, followed by a drastic decrease in T_(IM) for x = 0.15. Finally, the metal-insulator transition vanished for higher Cu concentrations (x = 0.20) in the observed temperature range. The variation in T_(IM) with the amount of added Cu is explained on the basis of the double exchange mechanism. Cu substitution has been found to perturb the Mn^(3+)-O-Mn^(4+) network and has a strong influence on T_(IM). The variation in resistivity maxima with temperature for different Cu concentrations showed a systematic trend up to x ∼ 0.10. We also explored the electrical conduction at T > T_(IM) in the framework of small polaron hopping in the adiabatic limit. The activation energy showed a strong dependence on the Cu concentration. The maximum magneto-resistance (MR) up to 10%, was observed for x = 0.15 at low temperatures.
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